tacoma 4wd will not work

I have a 98 tacoma ext. cab sr5 v6 since new...... now all of a sudden when i put it in 4wd either in high or low range it will only move about 1 foot in either direction then stops as if the is something locked. it will go just fine in 2wd but it will not go into 4wd on the go.... only when fully stopped and then will only move about 1 foot. PLEASE help me!